1. Field of the Invention
The invention relates in general to an amplifier, and more particularly to a differential amplifier and a method of improving stability thereof.
2. Description of the Related Art
Differential amplifier circuits are widely used in the electronics industry and are generally preferred over their single-ended counterparts because of their better common-mode noise rejection, reduced harmonic distortion, and increased output voltage swing. However, due to high impedance at the output nodes of the differential amplifier circuit, a common mode feedback (CMFB) control mechanism is often required in addition to well define the common mode voltage (Vcm). FIG. 1 shows a typical differential amplifier architecture. The differential amplifier architecture 100 includes an operational amplifier (OpAmp) 110, together with a plurality of resistors R1, R2, configured as an inverting type amplifier, with a CMFB loop 120. The CMFB loop 120 includes a common mode (CM) detector 122 and a compensation signal generator 124. The CM detector 122 senses the output common mode voltage based on the voltage outputs of the OpAmp 110, i.e., Vcm=(Von+Vop)/2. Then, utilizing the compensation signal generator 124, the common mode voltage Vcm is compared with a reference voltage Vref, and a corresponding error-correcting signal Vfb based on the differences between the Vcm and Vref is fed back to the biasing circuitry of the OpAmp 110 so as to compensate for and maintain the output common mode voltage level.
The CMFB loop 120 needs to be carefully designed, as the inherent parasitic effects of the circuit components introduce additional poles that degrade common mode stability. FIG. 2 shows a Bode plot illustrating the magnitude response of the differential amplifier circuit. As shown in the figure, the differential amplifier circuit usually possesses two significant poles P1 and P2, with the second pole P2 oftentimes being contributed by the unavoidable parasitic. As generally followed by designers as a rule of thumb, the differential mode feedback factor βDM is designed, so that the corresponding 1/βDM curve falls between the first pole P1 and the second pole P2, to render a stable differential feedback loop. For example, as shown in FIG. 2, the curve of 1/βDM=(R1+R2)/R1=2, by setting R1=R2, falls between P1 and P2. However, because in most of the cases the differential amplifier circuit has different differential feedback path and common mode feedback path, thus different differential and common mode feedback factors, a stable common mode feedback loop is not guaranteed merely by applying such a rule of thumb. For example, as shown in FIG. 2, the curve of 1/βCM, here equaling to 1 with βCM being the common mode feedback factor, falls below the second pole P2 rendering an unstable common mode feedback loop.
Two common approaches of fixing such common mode feedback loop instability during amplifier designed are illustrated in FIG. 3A and FIG. 3B. One can either design the amplifier so that the curve representing open loop gain shifts downwards, as shown by the dashed curve in FIG. 3A, to fit the common mode feedback factor curve between P1 and P2, or design the amplifier so that the common mode feedback factor curve shifts upwards, as shown by the dashed curve in FIG. 3B. The later approach shown in FIG. 3B is preferable because it does not induce potential additional input offset and noise problems, as the former approach shown in FIG. 3A may do.